Multi-stage scrubbing system and method for ethanol vent streams

CROSS-REFERENCE TO RELATED APPLICATION(S)

p-0002This application claims the benefit of U.S. Provisional Application Ser. No. 61/237,958, filed Aug. 28, 2009, the entirety of which is hereby incorporated by reference.

FIELD OF THE INVENTION

p-0003The present invention is generally directed to ethanol processing. More particularly, the present invention is directed to an improved system and method for extracting various organic compounds from a vent stream during ethanol production.

BACKGROUND OF THE INVENTION

p-0004Over the past thirty years, significant attention has been given to the production of ethyl alcohol, or “ethanol,” for use as an alternative fuel. Ethanol not only burns cleaner than fossil fuels but also can be produced using grains such as corn, which is a renewable resource. Further, the production of ethanol results in new sales outlets for corn, provides additional jobs, and reduces the nation's dependency on foreign oil.

p-0005Ethanol is typically produced from corn through either a wet or dry milling process. In the wet milling process, the corn kernel is separated into various components including germ, starch, protein, and fiber, resulting in several co-products. For example, separated germ may be further processed for oil recovery; starch may be saccharified and fermented for ethanol production; and protein and fiber may be used as feed material. In a dry mill process, whole corn is ground, treated with enzymes, and cooked. The resulting “mash” is treated with enzymes to further break down the starchy endosperm tissue into glucose. The converted mash is fermented and distilled, producing ethanol, carbon dioxide, and distiller's dried grains (“DDG”), which are the undissolved solid components (i.e. stillage) remaining in the fermentation tank after the broth is removed. Thus, DDGs are typically comprised of yeast and unfermented components of the corn.

p-0006The Dry grind process converts corn into two products, including ethanol and distiller's grains with solubles. If sold as wet animal feed, the co-product is known as distiller's wet grains with solubles (“DWGS”). Conversely, if dried for animal feed, the co-product is known as distiller's dried grains with solubles (“DDGS”). In the standard dry grind ethanol process, one bushel of corn yields approximately 8.2 kilograms (i.e. approximately 18 pounds) of DDGS in addition to the approximately 10.2 liters (i.e. approximately 2.7 gallons) of ethanol. These co-products provide a critical secondary revenue stream that offsets a portion of the overall ethanol production cost.

p-0007Typical ethanol plants employ scrubbers on the vent systems coupled to the fermentation vessels and other process gas discharges to minimize the discharge of vaporous ethanol and other volatile organic compounds (“VOCs”) from escaping into the atmosphere with the carbon dioxide and other vapors. As appreciated by those skilled in the art, VOCs may include compounds such as acetaldehyde, etheyl acetate, acrolein, and acetone. These scrubbers use fresh water along with a variety of additives to increase the solubility of the ethanol and VOCs. The precipitate of the scrubber is water with a low concentration of ethanol. This water mixture is then reintroduced into the process as make-up water which is then taken through the heating cycles along with the mash. It is in this area that the temperature is often taken over the flash point of ethanol, which in turn allows the ethanol to be lost to a vent system and destroyed in a thermal oxidizer. Consequently, both ethanol and VOCs are discharged into the atmosphere which reduces the ethanol yield and pollutes the environment.

p-0008Thus, what is needed is an improved system and method for extracting ethanol and VOCs from a vent stream during ethanol production.

BRIEF SUMMARY OF THE INVENTION

p-0009The present invention addresses the foregoing problems by providing a multi-stage scrubbing system that includes a pre-scrubber system structured to receive a vent stream therein and pre-scrub the vent stream with a pre-scrubber fluid. The pre-scrubber system includes a first outlet for discharging a pre-scrubbed vent stream and a second outlet for discharging a discharge stream of pre-scrubber fluid. A main scrubber system is fluidly coupled to the first outlet of the pre-scrubber system for receiving and further scrubbing the pre-scrubbed vent stream.

p-0010In accordance with another aspect of the present invention, a multi-stage scrubbing method is provided that includes the steps of directing a vent stream into a pre-scrubber system, pre-scrubbing the vent stream with a pre-scrubber fluid distributed by a fluid distributor, directing the pre-scrubbed vent stream from the pre-scrubber system into a main scrubber system, recirculating at least a portion of a fluid discharge stream from the pre-scrubber system back into the pre-scrubber system through the fluid distributor, scrubbing the pre-scrubbed vent stream with a main scrubber fluid, and recirculating at least a portion of a fluid discharge stream from the main scrubber system into the pre-scrubber system through the fluid distributor.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0011<figref idrefs="DRAWINGS">FIG. 1</figref> is a flow diagram illustrating a typical dry grind process for manufacturing ethanol.

p-0012<figref idrefs="DRAWINGS">FIG. 2</figref> is a circuit diagram illustrating one exemplary embodiment of a single-stage scrubbing system that operates in accordance with the dry grind process of <figref idrefs="DRAWINGS">FIG. 1</figref>.

p-0013<figref idrefs="DRAWINGS">FIG. 3</figref> is a circuit diagram illustrating one exemplary embodiment of a multi-stage scrubbing system in accordance with the present invention.

p-0014<figref idrefs="DRAWINGS">FIG. 4</figref> is a flow diagram illustrating several exemplary steps in a multi-stage scrubbing process.

p-0015<figref idrefs="DRAWINGS">FIGS. 5A and 5B</figref> are graphs illustrating the improved yield and pre-scrubber flow, respectively, for one exemplary processing plant having a multi-stage scrubbing system incorporated therein.

DETAILED DESCRIPTION OF THE INVENTION

p-0016Generally speaking, the present invention is directed to an improved system and method for extracting ethanol and VOCs from a vent stream during ethanol production. As will be described in detail below, a pre-scrubber has been developed that is operable to condense ethanol vapors from a vent stream prior to directing the vent stream to a main scrubber. Thus, the system and method of the present invention is a “multi-stage” scrubbing system. The pre-scrubbing step is accomplished by using low concentration ethanol-water from the main scrubber and circulating it through a pre-scrubber tower that condenses the vapors, thereby concentrating the ethanol. A heat exchanger may be employed that is operable to cool the circulated liquid and allow higher concentrations of ethanol to be obtained. The concentrated ethanol-water solution may be controlled at a level at or near the concentration of the beer feed. This stream of concentrated ethanol-water may then be sent directly to the distillation system where the ethanol is recovered as final product. As will be appreciated by those skilled in the art, the pre-scrubbing step prevents the loss of ethanol during ethanol production. The vapors that exit the pre-scrubber are directed to the main scrubber for final VOC removal before the vapors are exhausted to the atmosphere.

p-0017As stated above, one exemplary use of the system and method of the present invention is for extracting various organic compounds from a vent stream during the production of ethanol from a suitable grain, such as corn. However, the system and method of the present invention may be used for extracting selected organic compounds from vent streams during any agricultural or petroleum product processing without departing from the intended scope of the present invention.

p-0018<figref idrefs="DRAWINGS">FIG. 1</figref> is a flow diagram illustrating a typical dry grind ethanol process <b>10</b>. The process <b>10</b> begins at step <b>12</b> where the type of grain that will be used for ethanol production is selected. Because corn is the most common grain that is used for the production of ethanol, the dry grind process <b>10</b> will be described with reference to corn for purposes of example and not limitation. Although virtually any type and quality of corn may be used, one common type of corn is known as “No. 2 Yellow Dent Corn.” The “No. 2” refers to a quality of corn having certain characteristics as defined by the National Grain Inspection Association as generally known in the art. “Yellow Dent” refers to a specific type of corn as also generally known in the art. The industry average for ethanol yield in a dry grind plant is approximately 10.2 liters (i.e. approximately 2.7 gallons) of ethanol produced per 25.4 kilograms (i.e. one bushel) of No. 2 Yellow Dent Corn.

p-0019The process continues with a milling step <b>14</b> wherein dried whole corn kernels are passed through hammer mills in order to grind the kernels into meal or a fine powder. The ground meal is mixed with a suitable liquid, such as water, to create a slurry. Additionally, a commercial enzyme known as alpha-amylase may be added to the slurry. This slurry is then heated in step <b>16</b> to temperatures above 100° C. and allowed to cook in a pressurized “jet cooking” process. Jet cooking refers to a cooking process performed at elevated temperatures and pressures. The jet cooking process helps to break apart the starch granules present in the endosperm of the kernel, and the added enzymes function to break down the starch polymer into small fragments. Thus, the starch becomes gelatinized. The cooked corn mash is then allowed to cool and is held at a temperature between about 80° C. and about 95° C. in a liquefaction step <b>18</b>. During the liquefaction step <b>18</b>, additional alpha-amylase may also be added. As appreciated by those skilled in the art, holding the mash at the lower temperature allows the alpha-amylase to hydrolyze the gelatinized starch into maltodextrins and oligosaccharides (i.e. chains of glucose sugar molecules) to produce a liquefied mash or slurry.

p-0020The liquefaction step <b>18</b> is followed by a saccharification step <b>20</b> and a fermentation step <b>22</b>. As will be appreciated by those skilled in the art, these two steps may be combined and occur simultaneously in many commercial dry grind ethanol systems. When combined, these steps are referred to in the industry as “simultaneous saccharification and fermentation.” In the saccharification step <b>20</b>, the liquefied mash is first cooled to about 30-50° C. Then, a second enzyme known as glucoamylase is added. The addition of glucoamylase to the liquefied mash completes the breakdown of the starch into simple sugar (i.e. glucose). The saccharification step <b>20</b> may occur while the fermentation vessel is being filled with the mash in preparation for the fermentation step.

p-0021In the fermentation step <b>22</b>, yeast is added to the corn mash to begin the process of metabolizing the simple sugars into ethanol. As will be appreciated by those skilled in the art, a common strain of yeast known as <i>Saccharomyces cerevisae </i>is typically chosen due to its quick and efficient production of ethanol. The mixture of yeast and corn mash is fermented for approximately two days to allow the simple sugars to be converted into (1) ethanol and (2) carbon dioxide. The other components of the corn mash, including protein and oil, are substantially unchanged by the fermentation process. After fermentation, the fermentation mash (“beer”) will contain about 8-12% ethanol by weight, plus soluble and insoluble solids from the remaining grain components.

p-0022The carbon dioxide produced in the fermentation vessel may then be vented to and passed through a scrubber in step <b>24</b> to remove ethanol vapors and VOCs prior to releasing the stream to the atmosphere in step <b>25</b>. The scrubber typically utilizes fresh water along with a variety of additives to increase the solubility of the ethanol and VOCs. The precipitate of the scrubber is water with a low concentration of ethanol. This scrubber water may be recycled back into the process in step <b>26</b> by directing the water into the slurry tank where the ground corn meal is mixed with liquid to create the slurry. Alternatively, the scrubber water may be recycled back into the process at another suitable location, such as in a distillation tank during a subsequent distillation step.

p-0023The beer contained within the fermentation vessel is then sent to a distillation vessel in step <b>28</b> for distillation and dehydration. Optionally, the beer may be stored in a “beer well” prior to being sent to the distillation vessel. The beer well may be used to store the fermented beer and supply a controlled stream of beer to the distillation vessel at specified times. As appreciated by those skilled in the art, distillation is the process of separating mixtures based on differences in there volatilities in a boiling liquid mixture. Thus, distillation is a physical separation process and not a chemical reaction. At sea level, water vaporizes at 100° C. while alcohol vaporizes at about 73° C. This difference in vaporization temperature allows for the separation of water from ethanol upon heating the mixture in a distillation vessel.

p-0024More particularly, in the distillation step <b>28</b> fermented beer is pumped into the distillation vessel where it is boiled to vaporize the ethanol. The ethanol vapor is condensed in the distillation vessel, and liquid ethanol exits the top of the distillation vessel at about 95% purity (i.e. 190 proof) where it is directed into an ethanol purification unit at step <b>30</b>. The distillation process may also produce a vent stream comprised of carbon dioxide and ethanol, which may be directed through the scrubber along with the vapors produced in the fermentation step <b>22</b>. Thus, the ethanol that is scrubbed from both the fermentation and distillation vent streams may be recycled back into the process in any suitable manner as previously discussed.

p-0025In order to create an ethanol/gasoline blend, it is necessary to remove the roughly 5% of water that remains in the liquid ethanol. This may be accomplished by passing the 95% pure ethanol through a molecular sieve dehydration column to remove the residual water and create a final product that is almost 100% ethanol (i.e. roughly 200 proof). The resulting anhydrous ethanol is then ready to be used for motor fuel purposes. However, the anhydrous ethanol is commonly blended with another petroleum product, such as gasoline, prior to being used as a motor fuel.

p-0026The combination solid and liquid product that remains after the distillation step <b>28</b> is known as “whole stillage.” As appreciated by those skilled in the art, whole stillage may include, among other components, non-fermented starch and the protein, oil, and fiber components of the corn. Whole stillage is a valuable co-product of ethanol processing because it may be used as a feed ingredient for animals, which helps ethanol producers to offset the cost of ethanol production. Although the whole stillage may be used as a feed ingredient without further modification, it typically undergoes further processing prior to being fed to animals. In one exemplary method of processing the whole stillage, the stillage is sent through a centrifugation step <b>32</b> in order to separate the insoluble solids or “wet cake” from the liquid or “thin stillage.” After centrifugation, the thin stillage and wet cake may be used to create several types of feed, including DWGS at step <b>34</b> and DDGS at step <b>36</b>.

p-0027In order to produce the DWGS, the thin stillage may be sent to an evaporator to boil away moisture, leaving a thick syrup containing the dissolved solids from the fermentation step. This concentrated syrup may be mixed with the centrifuged wet cake, and the mixture may be sold as DWGS to animal feedlots. Alternatively, in order to produce the DDGS, the wet cake and concentrated syrup mixture may simply be dried with any suitable drying means and sold to animal feedlots.

p-0028<figref idrefs="DRAWINGS">FIG. 2</figref> is a circuit diagram illustrating one exemplary embodiment of a single-stage scrubbing system <b>50</b> that operates in accordance with the dry grind process <b>10</b> described in detail above with reference to <figref idrefs="DRAWINGS">FIG. 1</figref>. For purposes of simplicity, only a selected few components of the ethanol processing system are illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>.

p-0029Particularly, the scrubbing system <b>50</b> includes a fermentation vessel <b>52</b> containing the mixture of corn mash and yeast. The fermented corn mash, or beer, is then directed through an outlet <b>53</b> of the fermentation vessel <b>52</b> and pumped to a distillation vessel with a beer feed pump <b>54</b> where the ethanol is recovered in any suitable manner such as that previously described. A carbon dioxide vent stream <b>56</b> containing both ethanol vapors and VOCs is directed out of the fermentation vessel <b>52</b> through an outlet <b>58</b> and diverted through a suitable pipe system to an inlet <b>60</b> of a scrubber column <b>62</b>, such as a spray condenser or other suitable scrubbing means. As the carbon dioxide vent stream <b>56</b> travels up the scrubber column <b>62</b>, water from a suitable water source <b>64</b> is sprayed down the column in a counter-current fashion with a suitable fluid distributor <b>65</b>. As the water is being sprayed down from the top of the scrubber column <b>62</b>, it “wets” the inner surface and packing of the column and collects ethanol vapor, water vapor, and other pollutants that are escaping with the carbon dioxide vent stream <b>56</b> in the bottom of the column <b>62</b> as scrubber water. A suitable control means, such as a scrubber water return pump <b>66</b> and control valve <b>68</b>, are fluidly coupled to an outlet <b>67</b> in the bottom of the scrubber column <b>62</b> for controllably recycling a stream of scrubber water <b>70</b> back into the slurry tank where the ground corn meal is mixed with liquid to create the slurry. Alternatively, the stream of scrubber water <b>70</b> may be recycled back into the process at another suitable location, such as in a distillation vessel where the ethanol is distilled for recovery as final product. As appreciated by those skilled in the art, any suitable control means capable of recycling the stream of scrubber water <b>70</b> into another portion of the processing system may be used.

p-0030The forgoing scrubbing step results in a substantially clean oxide stream <b>72</b> that exits the scrubber column <b>62</b> through a suitable vent system <b>74</b> and is released into the atmosphere. Although the effectiveness of different scrubber columns will vary, the resulting oxide stream <b>72</b> is typically about 98% “clean.” As previously discussed, the batch of scrubber water produced by the scrubber column <b>62</b> is entrained with the carbon dioxide, ethanol and other organic compounds captured from the carbon dioxide vent stream <b>56</b>. In a typical system, the stream of scrubber water <b>70</b> may comprise approximately 2-5% ethanol.

p-0031As will be appreciated by those skilled in the art, the scrubber column <b>62</b> may comprise any suitable means for scrubbing ethanol and organic compounds from a vent stream. Examples of suitable scrubbing means include, but are not limited to, random packing, structured packing, or spray towers. Further, the fluid distributor <b>65</b> may comprise any suitable type of fluid distribution means for delivering the make-up water including, but not limited to, spray nozzles or a distributor that is operable to disperse a substantially continuous layer of water along the inner surfaces and packing of the scrubber column <b>62</b>.

p-0032As will further be appreciated by those skilled in the art, the various components of the exemplary scrubbing system <b>50</b> including the fermentation vessel <b>52</b>, beer feed pump <b>54</b>, scrubber column <b>62</b>, and scrubber water return pump <b>66</b> may be fluidly coupled together with any suitable piping system. The piping system may be formed from any suitable material that is capable of withstanding the temperature and pressure of the vent and fluid streams circulating through the system including, but not limited to, various metals, ceramics, plastics, fiberglass, or concrete. The requisite dimensions of the pipes in the piping system will vary depending upon numerous factors including, but not limited to, the size of the fermentation vessel, the amount of carbon dioxide vapor produced in the fermentation vessel, the volume and rate of the water cycled through the scrubber column, and the like. For example, with fermentation vessels having a capacity of about 5.5 million gallons, the portion of the piping system between the outlet <b>58</b> of the fermentation vessel <b>52</b> and the inlet <b>60</b> of the scrubber column <b>62</b> may be about 30 to 36 inches in diameter.

p-0033The piping system may be fluidly coupled to the various inlets and outlets of the scrubbing system <b>50</b> described above in any suitable, fluid-tight manner as will be appreciated by those skilled in the art. Exemplary coupling means may include, but are not limited to, welding, compression fit, or the like. Further, gaskets or other suitable sealing means may be disposed at the interface between a pipe and the component to which it is fluidly coupled in order to reduce or eliminate any sources of leakage.

p-0034Although ethanol processing systems that include a single scrubber column for scrubbing a carbon dioxide vent stream prior to being exhausted into the atmosphere are an improvement over prior systems that exhausted the carbon dioxide vent stream straight into the atmosphere, numerous problems still exist with such single scrubber systems. For example, although the single scrubber column <b>62</b> captures a large portion of the ethanol from the carbon dioxide vent stream in liquid form and recycles the liquid back into the process for use as make-up water for the corn mash slurry, when used in a subsequent cooking step the temperature of the slurry is driven up to about 85° C. which is above the flashpoint of ethanol. As a result, the ethanol is revaporized and lost through the vent system.

p-0035Further, as ethanol processing plants continue to push their processing rates (i.e. the quantity of ethanol being produced per unit of time) the single scrubber column becomes overworked and consequently cannot keep the exhaust stream from the column clean. As a result, many processing plants have turned to adding chemical additives such as sodium bisulfate or ammonium bisulfate to the make-up scrubber water to improve the absorption of VOCs during scrubbing. These chemical additives improve absorption by breaking down the surface tension of the scrubber water. However, this process adds expense to the overall production cost, which in turn drives up the price of ethanol for the consumer or reduces potential profits for the producer.

p-0036In summary, single-stage scrubbing systems such as the one illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref> feed a carbon dioxide vent stream containing organic compounds directly into a single scrubber column and rely on the affinity of the scrubbing water to the organic compounds contained in the vent stream to cause an entrainment of the organic compounds into the once-through scrubbing fluid. This entrained scrubber water is then recycled back into process or routed to another destination for ultimate disposal as waste. Although enhancement chemicals are sometimes added to the scrubbing water to enhance the affinity of the scrubbing water to the targeted organic compounds, these enhancement chemicals amount to excess chemical reactionaries that further contaminate discharge streams and do not have an overall positive impact on the ethanol production process.

p-0037As will be described in further detail to follow, the present invention is an improvement over such single-stage scrubbing systems and operates by cooling the carbon dioxide vent stream from the fermentation vessel (or other sources) prior to directing the vent stream into a main scrubber column. Consequently, the actual volume of recovered organic compounds is increased and not substantially eliminated as in previous scrubbing systems by other volatile means, such as by burning. Thus, the recovered organic compounds are extracted from the environment rather than discharged into the environment.

p-0038<figref idrefs="DRAWINGS">FIG. 3</figref> is a circuit diagram illustrating one exemplary embodiment of a multi-stage scrubbing system <b>100</b> in accordance with the present invention. Generally speaking, the multi-stage scrubbing system <b>100</b> may be structured as a combination of a single-stage scrubbing system, such as the scrubbing system <b>50</b> previously described, and a pre-scrubber system <b>102</b> operably incorporated therein. For purposes of clarity, the exemplary pre-scrubber system <b>102</b> of the present invention is shown bounded by a series of broken lines. Thus, one skilled in the art will appreciate that removal of the pre-scrubber system <b>102</b> from the multi-stage scrubbing system <b>100</b> may yield the single-stage scrubbing system <b>50</b> of <figref idrefs="DRAWINGS">FIG. 2</figref>.

p-0039As illustrated in <figref idrefs="DRAWINGS">FIG. 3</figref>, the exemplary multi-stage scrubbing system <b>100</b> includes the fermentation vessel <b>52</b>, beer feed pump <b>54</b>, scrubber column <b>62</b> (hereinafter referred to as the “main” scrubber column <b>62</b>), scrubber water return pump <b>66</b> (hereinafter referred to as the “main” scrubber water return pump <b>66</b>), and control valve <b>68</b> previously described with reference to <figref idrefs="DRAWINGS">FIG. 2</figref>. Additionally, the pre-scrubber system <b>102</b> generally includes a pre-scrubber column <b>104</b> having an inlet <b>106</b> and an outlet <b>108</b>, a pre-scrubber return pump <b>110</b>, a control valve <b>112</b>, a heat exchanger <b>114</b>, a chilled water source <b>116</b>, a blower <b>117</b>, a control valve <b>118</b>, and a fluid distributor <b>120</b>. Particularly, the pre-scrubber system <b>102</b> is configured such that the inlet <b>106</b> of the pre-scrubber column <b>104</b> is in fluid communication with the outlet <b>58</b> of the fermentation vessel <b>52</b>, the outlet <b>108</b> of the pre-scrubber column <b>104</b> is in fluid communication with the pre-scrubber return pump <b>110</b> and control valve <b>112</b>, the heat exchanger <b>114</b> is fluidly coupled to the chilled water source <b>116</b> for circulating chilled water therethrough, the blower <b>117</b> is fluidly coupled to a vent system <b>119</b> of the pre-scrubber column <b>104</b>, and the control valve <b>118</b> is in fluid communication with the main scrubber water return pump <b>66</b> for controlling flow of the stream of main scrubber water <b>70</b> from the main scrubber column <b>62</b> into the heat exchanger <b>114</b> for use as make-up water by the fluid distributor <b>120</b> in the pre-scrubber column <b>104</b>.

p-0040Although any suitable pre-scrubber column <b>104</b> may be used, the pre-scrubber column <b>104</b> may be substantially similar to the main scrubber column <b>62</b> previously described. However, as will be discussed in further detail to follow, the pre-scrubber column <b>104</b> may require several modifications to the internal components of the scrubber column. One modification is providing suitable components that allow for an increased amount of fluid flow through the pre-scrubber column <b>104</b> as compared to the main scrubber column <b>62</b>. Another modification is structuring the pre-scrubber column <b>104</b> such that at least a portion of the scrubber water collected at the bottom of the pre-scrubber column <b>104</b> may be recirculated through the heat exchanger <b>114</b> and used as cooled make-up water for the pre-scrubbing process.

p-0041The heat exchanger <b>114</b> generically illustrated in <figref idrefs="DRAWINGS">FIG. 3</figref> may comprise any suitable type of heat exchanger as will be appreciated by those skilled in the art including, but not limited to, a shell and tube heat exchanger, a double pipe heat exchanger, a finned pipe heat exchanger, a spiral pipe heat exchanger, a u-tube heat exchanger, a stacked plate heat exchanger, or the like. Further, any suitable flow design may be used such as parallel flow, counter flow, or cross flow. Although the heat exchanger <b>114</b> is illustrated as circulating chilled water from a chilled water source <b>116</b>, any suitable liquid or gas may be used without departing from the intended scope of the present invention. The cooling liquid or gas may maintain its phase throughout the cooling process, or alternatively may experience a change in phase during the cooling process.

p-0042Now that the exemplary components of the pre-scrubber system <b>102</b> have been described, the general operation of the multi-stage scrubbing system <b>100</b> will be discussed in detail. As illustrated in <figref idrefs="DRAWINGS">FIG. 3</figref>, instead of directing the carbon dioxide vent stream <b>56</b> containing both ethanol vapors and VOCs out of the fermentation vessel <b>52</b> directly into the main scrubber column <b>62</b>, the carbon dioxide vent stream <b>56</b> is directed into the inlet <b>106</b> of the pre-scrubber column <b>104</b> which is placed in front of and in series with the main scrubber column <b>62</b>. As the carbon dioxide vent stream <b>56</b> travels up the pre-scrubber column <b>104</b>, make-up water is sprayed down the column <b>104</b> in a counter-current fashion with the fluid distributor <b>120</b>. As the water is being sprayed down from the top of the pre-scrubber column <b>104</b>, it wets the inner surface and packing of the column <b>104</b> and collects ethanol vapor, water vapor, and other pollutants that are escaping with the carbon dioxide vent stream <b>56</b> from the fermentation vessel <b>52</b> as pre-scrubber water. The pre-scrubber water return pump <b>110</b> and the control valve <b>112</b> are fluidly coupled to the outlet <b>108</b> of the pre-scrubber column <b>104</b> and are operable for controllably recycling a stream of pre-scrubber water <b>130</b> to various other portions of the system. Particularly, the pre-scrubber water return pump <b>110</b> and control valve <b>112</b> direct a first portion <b>132</b> of the stream of pre-scrubber water <b>130</b> to distillation where the ethanol is recovered as final product, and a second portion <b>134</b> of the stream of pre-scrubber water <b>130</b> to the heat exchanger <b>114</b> where it merges with a second portion <b>138</b> of the stream of main scrubber water <b>70</b> to form the make-up water for the pre-scrubber column <b>104</b>. A first portion <b>136</b> of the stream of main scrubber water <b>70</b> is directed to the slurry tank, distillation vessel, or the like as will be described below.

p-0043In one exemplary embodiment the heat exchanger <b>114</b> is operable to cool the combined second portions <b>134</b> and <b>138</b> of the scrubber water streams to a temperature within a range between about 16° C. and about 27° C. Any suitable ratio of pre-scrubber water to main scrubber water in the combined stream may be used. As will be appreciated by those skilled in the art, the ratio may be controlled and modified as necessary with the control valves <b>112</b> and <b>118</b>.

p-0044The stream of pre-scrubber water <b>130</b> exiting the outlet <b>108</b> of the pre-scrubber column <b>104</b> is preferably maintained at or near the ethanol level of the beer feed from the fermentation vessel <b>52</b>. Exemplary ethanol levels of the stream of pre-scrubber water <b>130</b> may be in a range between about 12% and about 15%, although higher or lower levels are also contemplated. The pre-scrubbing system <b>102</b> may operate to maintain the stream of pre-scrubber water <b>130</b> at or near the ethanol level of the beer feed with any suitable mass or density flow meter, such as a Coriolis type mass or flow meter. Such “proof meters” are known to those skilled in the art, and may be operably coupled to a controller that is programmed with data tables that relate, for example, temperature, density, and concentration data. Alternatively, the ethanol concentration may be monitored and controlled manually using a hydrometer and temperature reference chart. Various other suitable means for maintaining a desired ethanol concentration in the stream of pre-scrubber water <b>130</b> are also possible as will be appreciated by those skilled in the art.

p-0045Generally speaking, make-up water is circulated through the pre-scrubber column <b>104</b> at a greater flow rate/volume than the make-up water circulated through the main scrubber column <b>62</b>. In one exemplary embodiment, the flow through the pre-scrubber column <b>104</b> may be about 10 times greater than the flow through the main scrubber column <b>62</b>.

p-0046As further illustrated in <figref idrefs="DRAWINGS">FIG. 3</figref>, a pre-scrubber vent stream <b>140</b> exits the pre-scrubber column <b>104</b> through the vent system <b>119</b> and is directed to the inlet <b>60</b> at the bottom of the main scrubber column <b>62</b>. Optionally, a fan or blower unit such as the blower <b>117</b> may be operably coupled to the portion of the piping system between the pre-scrubber column <b>104</b> and the main scrubber column <b>62</b> to blow the pre-scrubber vent stream <b>140</b> into the main scrubber column <b>62</b>. The blower <b>117</b> is preferably operable to maintain a steady, desirable flow rate of the vent stream <b>140</b> into the main scrubber column <b>62</b>. Any suitable fan or blower unit that is operable to transfer a substantially steady stream of vapors between the pre-scrubber column <b>104</b> and the main scrubber column <b>62</b> may be used including, but not limited to, a centrifugal fan, a positive displacement blower, an eductor, or the like. The blower <b>117</b> may preferably include a means for adjusting the flow rate/volume to any desired level.

p-0047As the pre-scrubber vent stream <b>140</b> travels up the main scrubber column <b>62</b>, make-up water is sprayed down the column in a counter-current fashion with the fluid distributor <b>65</b>. As the water is being sprayed down from the top of the main scrubber column <b>62</b>, it collects the majority of the ethanol vapor, water vapor, and other pollutants that were not collected by the pre-scrubber column <b>104</b> during the pre-scrubbing stage. After the “second” stage of scrubbing, the concentration of ethanol in the stream of scrubber water <b>70</b> may be about 1% or less. The main scrubber water return pump <b>66</b> and control valve <b>68</b> are fluidly coupled to the outlet <b>67</b> in the bottom of the column <b>62</b> for controllably recycling the first portion <b>136</b> of the stream of main scrubber water <b>70</b> into a slurry tank, distillation vessel, or the like, and for injecting the second portion <b>138</b> of the stream of main scrubber water <b>70</b> into the second portion <b>134</b> of the stream of pre-scrubber water near the inlet of the heat exchanger <b>114</b> as described above to form the pre-scrubber make-up water. Thus, substantially any amount of ethanol that happens to slip through the pre-scrubber column <b>104</b> is picked-up by the main scrubber column <b>62</b> and recycled back into the pre-scrubber <b>104</b> as make-up water. This ensures that the multi-stage scrubbing system <b>100</b> picks-up almost 100% of the ethanol for recovery as final product.

p-0048Because the pre-scrubber column <b>104</b> operates at a much higher flow rate/volume than the main scrubber column <b>62</b>, the second portion <b>138</b> of the stream of scrubber water <b>70</b> typically accounts for a small percentage of the combined pre-scrubber make-up water. For example, the second portion <b>138</b> of the stream of main scrubber water <b>70</b> may account for approximately 5-10% of the total volume of pre-scrubber water, with the second portion <b>134</b> of the stream of pre-scrubber water <b>130</b> accounting for the other 90-95%. Obviously, these percentages may vary depending upon the precise flow rates in the scrubber columns and are presented merely for purposes of example and not limitation.

p-0049Due to the presence of the pre-scrubber column <b>104</b>, the concentration of ethanol and VOCs in the vent stream <b>140</b> that is received by the main scrubber column <b>62</b> is greatly reduced. As a result, the load on the main scrubber column <b>62</b> is also much less than it would otherwise be if receiving the carbon dioxide vent stream <b>56</b> directly from the fermentation vessel <b>52</b>. From a mechanical standpoint the vapors are cooler, potentially on the order of about 15-25° C. cooler, which corresponds with a reduced specific volume of the vapors as well as reduced vapor velocities in the main scrubber column <b>62</b>.

p-0050In view of the foregoing, the main scrubber column <b>62</b> receives a vent stream <b>140</b> having vapors that are smaller, compressed, and much cooler than the vapors in the carbon dioxide vent stream <b>56</b>. It follows that the reduced loading reduces the amount of make-up water required by the main-scrubber column <b>62</b> and minimizes or eliminates the need for using chemicals in the make-up water to “clean-up” the oxide stream <b>72</b>.

p-0051As will be appreciated by those skilled in the art, the pre-scrubber system of the present invention may include additional or different components without departing from the intended scope of the invention. Thus, the exact configuration of the pre-scrubber system <b>102</b> set forth in <figref idrefs="DRAWINGS">FIG. 3</figref> is illustrated and described merely for purposes of example and not limitation. Further, the pre-scrubber system of the present invention may be incorporated into any ethanol processing and scrubbing system as will be appreciated by those skilled in the art without departing from the intended scope of the present invention. Therefore, the scrubbing system <b>50</b> is described and illustrated in combination with the pre-scrubber system <b>102</b> merely for purposes of example and not limitation.

p-0052As previously discussed with respect to the scrubbing system <b>50</b> of <figref idrefs="DRAWINGS">FIG. 2</figref>, any suitable piping system may be used to fluidly couple the various components of the multi-stage scrubbing system <b>100</b> together with departing from the intended scope of the present invention. Thus, exemplary coupling means such as welding and compression fit may be used to fluidly couple together pipe sections formed from suitable materials such as various metals, ceramics, plastics, fiberglass, or concrete.

p-0053<figref idrefs="DRAWINGS">FIG. 4</figref> is a flow diagram illustrating several exemplary steps in the multi-stage scrubbing process described above with reference to <figref idrefs="DRAWINGS">FIG. 3</figref>. The multi-stage scrubbing process <b>200</b> begins with step <b>202</b> where a carbon dioxide vent stream is directed into a pre-scrubber column. The process continues with step <b>204</b> where the carbon dioxide vent stream is scrubbed in the pre-scrubber column with pre-scrubber make-up water to remove ethanol vapor, water vapor, and other pollutants that are escaping with the carbon dioxide vent stream. Following the pre-scrubbing step <b>204</b>, a pre-scrubber vent stream is vented from the pre-scrubber column into a main scrubber column at step <b>206</b>.

p-0054Simultaneously with the venting step <b>206</b>, at least a portion of the pre-scrubber water collected in the bottom of the pre-scrubber column is recirculated back toward the pre-scrubber column for use as one component of the pre-scrubber make-up water in step <b>208</b>. Once in the main scrubber, the pre-scrubber vent stream is scrubbed to remove substantially all remaining ethanol vapor, water vapor, and other pollutants in step <b>210</b>. The process continues at step <b>212</b> where a substantially clean oxide stream is vented from the main scrubber.

p-0055Simultaneously with the venting step <b>212</b>, at least a portion of the main scrubber water collected in the bottom of the main scrubber column is recirculated back toward the pre-scrubber column where it is mixed with the pre-scrubber water from step <b>208</b> for use as a second component of the pre-scrubber make-up water in step <b>214</b>. The pre-scrubber make-up water (which is a combination of “exhausted” water from the pre-scrubber and main scrubber) is cooled within a heat exchanger in step <b>216</b>, and the process <b>200</b> repeats until the carbon dioxide vent stream terminates.

p-0056Although several exemplary steps were described with reference to the multi-stage scrubbing process <b>200</b>, those skilled in the art will appreciate that the order and number of steps may be modified without departing from the intended scope of the present invention. Thus, the exemplary steps were provided merely for purposes of example and not limitation.

p-0057Now that the general structure and operation of a multi-stage scrubbing system in accordance with the present invention has been described, the experimental results from one exemplary ethanol processing plant incorporating a multi-stage scrubbing system will be provided with reference to <figref idrefs="DRAWINGS">FIGS. 5A and 5B</figref>. Particularly, a pre-scrubber system (similar to the pre-scrubber system <b>102</b>) was incorporated into a 40 million gallon/year ethanol processing facility having a single-stage scrubbing system (similar to the single-stage system <b>50</b>) to achieve a multi-stage scrubbing system that resembles system <b>100</b>.

p-0058<figref idrefs="DRAWINGS">FIG. 5A</figref> is a graph illustrating yield of the processing plant over a period of 29 days in gallons of ethanol per bushel of corn. Particularly, line Y<b>1</b> depicts the yield of the processing plant using only the single-stage scrubbing system, while line Y<b>2</b> depicts the yield of the processing plant using the multi-stage scrubbing system in accordance with the present invention. Over the 29 day test period, the single-stage scrubbing process resulted in an average yield of about 2.65 gallons/bushel, while the multi-stage scrubbing process of the present invention resulted in an average yield of about 2.69 gallons/bushel. Thus, the ethanol processing plant realized an improvement in yield of about 1.51% using the multi-stage scrubbing system and method of the present invention.

p-0059<figref idrefs="DRAWINGS">FIG. 5B</figref> is a graph illustrating the pre-scrubber flow rate (in gallons per day) that was required to achieve the average yield of about 2.69 gallons/bushel. Over the 29 day test period, the average daily pre-scrubber flow rate was about 1,325 gallons/day.

p-0060Those skilled in the art will appreciate that the improved yield and required pre-scrubber flow rate may vary based upon numerous factors such as the size of the facility (i.e. the amount of ethanol produced), the effectiveness of the main scrubber, the temperature of the pre-scrubber water, and the like. However, theoretical models and other experimental results have shown that improvements in yield even greater than about 1.5% are possible with the multi-stage scrubbing system and method of the present invention.

p-0061The system and method of the present invention have been described with reference to scrubbing vent streams from a fermentation vessel merely for purposes of example and not limitation. Thus, the multi-stage scrubbing system may be used to scrub vent streams from any source without departing from the intended scope of the present invention.

p-0062Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.